There has been much interest in recent years in nonlinear interactions in optical fibres. The low nonlinearity of, for example, silica glass is offset by long interaction lengths and high power density in fibre to yield spectacular nonlinear effects. For most nonlinear processes the physical fibre length can be made longer than the effective interaction length, which is governed by phase matching, pulse broadening, walk-off and attenuation. In particular, the fibre dispersion plays a key role in short pulse propagation and in phase matching conditions for nonlinear processes.
In the spectral region beyond 1300 nm, where the material dispersion of silica glass is itself anomalous, fibres can be designed and made to have a modal dispersion which is normal or anomalous, with a zero dispersion at any given wavelength (for example the dispersion shifted fibres used in telecommunications systems). It is not possible, however, to move the zero dispersion wavelength, λ0, of a silica step-index single-mode optical fibre to wavelengths shorter than 1270 nm, the zero dispersion wavelength of bulk silica.
Photonic crystal fibres (PCFs, also known as micro-structured fibres or holey fibres) are a relatively new kind of optical fibre. A PCF comprises a cladding region, formed from solid matrix material defining a plurality of elongate holes, and a core region. PCFs can guide light in their core region by a number of mechanisms, including total internal reflection at the interface between the core and cladding region. Even if a PCF is made from a single solid material, the holes in the cladding region lower the effective refractive index of the cladding, providing a refractive index step between a solid core and the cladding region and enabling total internal reflection of guided light. In photonic crystal fibres, it is possible to shift the zero dispersion wavelength of single-mode silica fibres to much shorter wavelengths (See for example D. Mogilevtsev, T. A. Birks and P. St. J. Russell, “Group-velocity dispersion in photonic crystal fibres,” Opt. Lett., 23 (21), 1662-1664 (1998), J. C. Knight, J. Arriaga, T. A. Birks, A. Ortigosa-Blanch, W. J. Wadsworth, P. St. J. Russell, “Anomalous dispersion in photonic crystal fibre,” IEEE Photonic Technology Letters, 12, 807-809 (2000) and J. K. Ranka, R. S. Windeler and A. J. Stentz: “Visible continuum generation in air-silica microstructure optical fibres with anomalous dispersion at 800 nm,” Opt. Lett, 25 (1), 25-27 (2000)). This has been exploited to dramatic effect in supercontinuum generation in small-core, high-index contrast PCF with zero dispersion wavelengths in the region 580-900 nm pumped with modelocked Ti:sapphire lasers at 750-850 nm. Though these fibres are typically not strictly single-mode, higher order modes are difficult to excite and are also not coupled to the fundamental mode by normal bending, so the fibres may be used as if single mode.
Not only can strictly single-mode PCFs be fabricated, but also so-called endlessly single-mode PCFs which support only one guided mode over all wavelengths (See for example T. A. Birks, J. C. Knight, and P. St. J. Russell, “Endlessly single-mode photonic crystal fibre,” Opt. Lett. 22, 961-963 (1997) and T. A. Birks, D. Mogilevtsev, J. C. Knight, P. St. J. Russell, J. Broeng, P. J. Roberts, J. A. West, D. C. Allan, and J. C. Fajardo, “The analogy between photonic crystal fibres and step index fibres,” Optical Fibre Conference, Paper FG4-1, pages 114-116, Friday, Feb. 26, 1999).
Schreiber et al. describe at pp 71-78, Opt. Comm. Vol. 228 (2003) generation of a supercontinuum spectrum from PCF by pumping with picosecond pulses from an ytterbium-doped fibre amplifier operating at 1040 nm.
Town et al. describe at pp 235-238, Appl. Phys. B—Lasers and Optics, Vol. 77 (2003) generation of a supercontinuum spectrum from a randomly micro-structured air-silica optical fibre, by pumping with nanosecond pulses from a Q-switched Nd:YAG laser.
Coen et al. describe at pp 1356-1358, Opt. Lett., Vol. 26 (2001) generation of a supercontinuum from a PCF by pumping with 60 ps pulses at 675 W peak power from a Kr-ion laser operating at 647 nm.
Dudley et al. describe at pp 765-771, J. Opt. Soc. Am. B, Vol. 19 (2002) generation of a supercontinuum from air-silica micro-structured fibre by pumping with pulses of 0.8 ns duration from a frequency-doubled, Q-switched Nd:YAG microchip laser operating at 532 nm.